1. Field of the Invention
The invention relates to a method for generating a plasma jet under atmospheric pressure for the surface treatment of workpieces, the plasma jet being generated by electrically exciting a plasma-capable medium by means of an electric arc periodically ignited between a cathode and an anode constructed as a nozzle, and the plasma jet being directed through the nozzle in the direction of the surface to be treated.
Furthermore, the invention relates to a device for generating a plasma jet under atmospheric pressure for the surface treatment of workpieces, having a supply line for the plasma-capable medium, an apparatus for electrically exciting the plasma-capable medium by means of an electric arc periodically ignited between a cathode and an anode constructed as a nozzle.
2. Description of the Related Art
A plasma is understood to mean a gas, which contains free charge carriers, which is why electrically conductive gases are also often spoken of. Plasmas can be used in different ways, depending on pressure and temperature, for example in gas discharge lamps, in material deposition, for analysis, but also for processing workpieces and for disinfecting objects, body parts or wounds. With regards to the pressure of the plasma gas, a difference is made between low-pressure plasmas, normal-pressure plasmas or atmospheric-pressure plasmas and high-pressure plasmas. The subject invention is definitively directed towards the generation of plasma jets under atmospheric pressure at relatively low temperature and primarily conceived for uses for treating surfaces of workpieces or the like. The treatment of surfaces of workpieces with plasmas in low-pressure chambers is an already well-established and known method, which has already been used industrially for many years. Low-pressure plasmas stand out owing to good gap-penetration and high effectiveness. They are well-suited for treating small parts and also for bulk material. In addition to high investment costs, the required process time for draining the plasma chambers and the lack of an option for what is known as in-line execution of the plasma treatment are disadvantageous. The treatment of larger components also rapidly becomes uneconomical owing to the large vacuum chambers required therefor in the case of low-pressure plasmas.
Uses of atmospheric-pressure plasmas were primarily limited solely to what are known as thermal plasmas, which were used at plasma temperatures of a few 1000° C. for melting processes (such as e.g. welding, soldering, cutting, etc.). More recent developments now also allow the realization of what are known as non-thermal atmospheric-pressure plasmas, which also allow uses at lower plasma temperatures of a few 10° C. up to a few 100° C. Thus, novel interesting fields of application are being opened up for surface treatment by means of atmospheric-pressure plasmas, such as e.g. the cleaning and activation of materials or coating by means of plasma polymerization through to medical uses, such as the disinfection of open wounds.
A special gas is usually used for generating an atmospheric-pressure plasma (e.g. processed air, N2, helium, argon), which is brought to the plasma state in a plasma burner by supplying energy. This plasma is blown onto the surface to be treated via a nozzle, whilst the plasma burner is moved over the workpiece with a defined spacing and speed. In this case, there are interactions of the plasma with the ambient air (quenching, mixing in of air molecules owing to the turbulent flows) and interactions with the surface to be treated. By far the most convincing advantage of this concept is the in-line capability and the simple integration of the process into existing production chains resulting therefrom. There are limitations when treating bulk material and small parts, insofar as the same cannot be placed in front of the plasma burner to a satisfactory extent. The oxidative effect of these plasmas is also undesired in some uses.
Due to the interaction of the plasma jet with the surface of the workpiece, the same is activated and/or cleaned. The cleaning effect is based, inter alia, on mechanical processes, owing to the bombardment of the surface with excited molecules, atoms and ions and also on chemical processes. In this case, the contaminants on the surface react with the excited particles in the plasma to form gaseous reaction products, which are finally removed by the gas flow. Uses of this process are to be found in microfine cleaning and the removal of oils, fats, silicones, oxides, fibres or thin coatings. A targeted changing of the surface structure (e.g. microstructuring) is also possible.
The activating effect of the plasma is achieved by chemical surface reactions. Depending on the base material and the composition of the plasma, radical locations or chemically-active groups (e.g. hydroxyl or carboxyl groups) are created on the surface. These effect a change of the surface energy and as a consequence a change of the surface properties of the treated workpieces. Thus, for example the wetting behavior of materials can be influenced in a targeted fashion. An originally hydrophobic surface (such as e.g. polypropylene) can be brought into a hydrophilic state by plasma activation. Both effects, the formation of chemically active groups and the change of the surface energy, can drastically improve the wetting behavior, layer formation and adhesion of coatings.
Many uses of the processes just described are to be found in the pretreatment of materials before bonding, soldering, welding, adhesive bonding, painting, printing and coating. The cleaning and activation of materials by means of atmospheric-pressure plasmas can replace chemical cleaning processes and the use of primers. The dispensing with solvents or other chemicals, which is connected therewith, makes plasma technology both economically and ecologically interesting.
By way of example, EP 0 761 415 B9 relates to a method for increasing the wettability of the surface of workpieces using atmospheric-pressure plasma, the plasma being generated by means of an electric arc excited at high frequency, which arc does not exit from the nozzle of the plasma burner.
A method for operating a steam plasma burner and a steam cutting device has become known from EP 1 922 908 B1, from which an optimized switchover from a transmitted mode to a non-transmitted mode for an optimized processing result emerges.
AT 510 012 B1 describes a steam plasma burner with an inductive heater for vaporising water or other liquids as plasma-capable medium. The steam plasma burner described therein can be used for cutting workpieces for example.
It is disadvantageous in the case of previous methods and devices for generating plasma jets, that at high activation of the plasma, an increase in the plasma temperature also results, for which reason specially activated plasmas are not suitable for all uses, for example surface pretreatment of sensitive workpieces.